308 PROCEEDINGS OF THE AMERICAN ACADEJIY. 



discharge is kept low the conductivity of the gap is not maintained 

 over the interval of zero current by the presence of incandescent metallic 

 or carbon vapor, and the high resistance of the gap is partially estab- 

 lished at each crossing of the axis by the current curve. This necessi- 

 tates a reionization of the gas at the beginning of each current loop, or 

 the breaking down of a possible oxide film, thereby distorting the wave 

 in the neighborhood of the axis, as is shown by the figures. 



The general form of the curve for spark damping, which is indicated 

 in the photographs, was at times momentarily clearly outlined on the 

 fiuorescent screen and is shown by the drawing of Figure 18. Ap- 

 parently, in some of the photographs, the loops begin, in point of 

 time, before the ending of the previous loop. This apparent paradox 

 is due to the slight shift in the time axis resulting from the reaction 

 of the strong secondary discharges upon the potential of the primary 

 condenser. 



The oscillogram photographs of spark damping here shown were 

 taken for aluminum terminals because of the greater regularity of the 

 discharge between terminals of that metal. Many other metals were 

 tried, such as copper, cadmium, zinc, etc., and in practically every case 

 in which the discharge was a spark -discharge, the oscillations took the 

 same form as is shown in Figure 18. 



When one of the terminals is carbon or magnesium, the current loops 

 on one side of the time axis are of the pure arc form, while those on 

 the other side are of the spark-discharge form. 



If the terminals are dissimilar there is usually evidence of a rectify- 

 ing effect. Particularly is this so if one terminal is of aluminum and 

 the other of copper or iron. Cut d of Plate 7 shows this rectification. 



The effect of hydrogen on the discharge is indicated by cuts e and 

 / of Plate 7, which were taken under the same conditions except for 

 the H surrounding the gap terminals in the case of cut/ The hydro- 

 gen serves, in this case, principally to ensure a spark rather than an arc 

 discharge, and probably increases the damping. 



The outward appearance of the discharge is in no way an indication 

 as to whether the oscillogram takes the arc or spark form. The form 

 of the oscillogram serves conveniently to differentiate between the 

 pure arc and the spark discharges, when the terms arc and spark are 

 applied to discharges operating on alternating currents. The discharge 

 of the Cu-Al oscillation gap, which has been studied in this investiga- 

 tion, cannot, according to this definition, be called an arc, but is a 

 spark discharge. 



The photographs here shown of gap damping furnish in no way a 

 complete discussion of the subject, but are presented to give additional 



